Thermal fuse

ABSTRACT

A thermal fuse is disclosed in which a bimetallic snap disc operates an elongated cutter causing the cutter to engage and rupture a thin metal conductor. Terminal elements provided with sawtooth-like teeth engage the side of the conductor opposite the cutter to provide points of high localized stress which facilitate the cutting or rupturing operation.

1 States atent 1 1 3,715,697

' 1 51 Feb. 6, 1973 54 THERMAL FUSE 3,317,693 5/1967 Bolesky ..337 354 x3,164,702 1/1965 Ruckriegel et al. ..337/346 [75] Invent w d T MansfieldO 2,531,115 11/1950 Doak ..337/333 [73] Assignee: Therm-O-Disc,Incorporated, Mansfield, Ohio Primary Examiner-Bernard A. GilheanyAssistant ExaminerF. E. Bell [22] Filed May 1971 1 Att0rneyMcNenny eta1. [21] Appl. No.: 146,143

' [57] 1 ABSTRACT [52] US. Cl. 337/1 57, 33 7/3,'337/333' thermal'fuse idisclbsed in which a bimetallic S [51] llilt. Cl. ...H01h 37/00 I discoperates an elongated cutter causing the cutter to [58] Fleld of Search..337/157, 3, 33 3, 354, 346 engage and rupture'a thin metal conductorTerminal q elements provided with sawtooth-like teeth engage the [5References t v, side of the conductor opposite the cutter to provideUNITED STATES PATENTS points of high 1ocz xlized stress which facilitatethe I 1 cuttlng or rupturmg operanon. 2,934,622 4/1960 Massar "337/1573,517,366 6/1970 Patrichi v .....33 7./157 X 13 Claims, 5 DrawingFigures PATENTEDFEH 6 I975 INVENTOR.

THERMAL FUSE BACKGROUND OF THE INVENTION This invention relatesgenerally to fuse devices and more particularly to atemperature-responsive fuse having a frangible element which is rupturedto break a circuit when a predetermined temperature exists.

PRIOR ART Electrical fuse devices are often employed to break a circuitwhen a predetermined condition is reached in which damage to the systemmay occur or dangerous conditions may prevail. In many instances, thefuses are current-responsive. Such fuses, when the condition to besensed is current flow, normally employ a conductor which melts when apredetermined current is carried thereby. In other instances, fuses areemployed which are not sensitive to current flow, but rather totemperature. Such fuses often are provided with a material which meltsat a given temperature and causes a circuit to be broken. An example ofsuch a fuse is illustrated in the United States Letters Patent Number3,155,800.

SUMMARY OF THE INVENTION A fuse in accordance with the present inventionemploys a frangible conductor in combination with a condition-responsivemechanism which operates to rupture the conductor and break the circuitwhen a predetermined condition occurs. In the illustrated embodiment,the fuse is temperature-responsive and utilizes a bimetallic snap discto actuate a cutter element which functions to cut a conductor and breakthe circuit. Such device provides reliability and low cost because ofits simplicity.

BRIEF DESCRIPTION OF THE DRAWINGS I FIG. 1 is a plan view of a thermalfuse incorporating the presentinvention;

FIG. 2 is across section taken generally along 2-2 of FIG. I;

' FIG. 3 is a cross section taken generally along 3 3 of FIG. 1illustrating the mechanism before operation;

FIG. 4 is a view similar to FIG. 3, but illustrating the mechanism afteroperation; and

rivets I3 and 14 which press the ends of the conductor strip 18 betweenthe terminals and the adjacent surface of the body 10.

Thermal-responsive means are provided to rupture the conductor strip 18when a predetermined temperature condition is reached. Such meansinclude a bimetallic snap disc 19 and a cutter element 24. The disc 19is mounted at its periphery between a disc retainer 21 and the end face22 of the body 10. The disc retainer 21 is recessed at its center at 23to provide clearance for the disc 19 when the disc is in the normal orunoperated position of FIGS. 2 and 3.

The cutter element 24 is guided for longitudinal movement in the body 10by guide surfaces 26 and is engagable at one end 27 with the center ofthe disc 19. The other end of the cutter element 24 is provided with asloping end face 28 which extends to an edge 29 extending generallyperpendicular to the length of the strip 18.

The snap disc 19 is formed with a shallow dish shape and has twopositions of stability between which it moves with snap action when thedisc reaches predetermined temperatures, the value of which depends uponthe shape and'material of the disc. Prior to the operation of thedevice, the disc 19 is dished away from the conductor strip 18 asillustrated in FIGS. 2 and 3. This position is referred to herein as thenormal or unoperated position of the disc.

When the disc reaches the operating temperature of the device, it moveswith snap action from the normal position of FIGS. 2 and 3 to theoperated position of FIG. 4 in which the center of the disc is curvedtoward the conductor strip 18. The cutter element 24 is sized so thatwhen the disc is in the normal position'the edge 29 is spaced slightlyfrom the conductor strip 18 and so that no force of any significance isapplied to the conductor strip 18. The cutter element 24 is preferablymolded of a non-conducting material such as phenolic resin and issufficiently light in weight so that it will not damage the conductorstrip 18 so long as the disc FIG 5 is an enlarged fragmentaryperspective 'view illustrating the conductor and cutter .member prior totheoperation ofthe device. 1

DETAILED DESCRIPTION OF THEDRAWINGS The illustrated embodiment of thisinvention is provided with'a base 10 preferably. molded from anonconducting material such as a phenolic resin. First and secondterminal elements 11 and 12 are secured to the leads in a system to beprotected by the device. The inner ends of each terminal element isserrated to pro-' vide a plurality of sawtooth shaped teeth 17 extendingthe'reacross ln the illustrated embodiment, each terminal is provided'with three teeth. A thin conductor strip 18 extends between the twoterminals 11 and 12 and provides an electrical connection therebetween.The strip 18 is sized to extend along .each of the ter-.

minalsand is secured to the terminals at its ends by the I remains inits normal position even if the device is mounted in an invertedposition so that the edge 29 lays against the conductor strip.

The operation of the snap disc 19 to the operated position, however,causes the edge 29 to engage the conductor strip 18 and exert sufficientlateral force on the conductor strip to causeit to rupture asillustrated in FIG. 4. The various elements are proportioned so that thecutting edge 29 engages the side of the strip 18 opposite the terminalssubstantially adjacent to the ends of the teeth 17 on one of theterminal elements. In the illustrated embodiment, such engagement issubstantially adjacent to the ends of the teeth on the terminal 11.

The teeth 17 are preferably formed by a shearing operation which leavesa burr or sharp edge (not illustrated) along the edges of the teethadjacent to the conductor strip 18. Such sharp edges facilitate therupture of the conductor strip when the device operates. Since the edgeengages the flat side of the conductor strip 18 substantially adjacentto the ends of the teeth 17- on the terminal element 11, the strip ispressed laterally against the points of such teeth which produce highlocalized stress and perform the initial penetration. Continued movementof the cutter element 24 causes the strip material to shear back alongthe teeth from the points until the strip is completely cut or ruptured.The various elements are proportioned so that as the disc moves to theoperated position of FIG. 4, the sloping surface 28 bends the end of thestrip remaining attached to the terminal 12 away from the terminal 11 toinsure that the electrical connection is completely broken.

Preferably, the disc 19 is formed so that it will not snap back to itsnormal position under any normal environmental temperature encountered.With this structure, the disc remains in the operated position and holdsthe cutter in between the two cut ends of the strip. However, it is alsopreferable to form the conductor strip of a material which issufficiently ductile so that it permanently deforms or bends duringoperation and remains in the bent or separated position even if the discsnaps back to its normal position allowing the cutter to return to itsunoperated position.

The strip 18 may be formed of copper. However, where substantialcurrents are required, conductor strips are preferably formed of silver.The width of the strip and the thickness is sized so that the currentsencountered by the device will not cause any appreciable heating due tocurrent flow. Consequently, the device is operated by the temperaturesensed by the disc 19 and is not sensitive to current flow to anyappreciable extent.

In the illustrated embodiment, the cutter element 24 is symmetricalabout its longitudinal center excepting at the cutter end and provides acentral section which is generally cylindrical and opposed axiallyextending projections 31 which are guided by mating surfaces 32 formedin the body to insure that the edge 29 extends perpendicular to thelength of the conductor 18. The two terminals 11 and 12 are symmetricalwith respect to the center axis of the device so the cutter element 24can be installed in either of two opposite positions.

For example, in the illustrated position, the cutting edge 29 issubstantially adjacent to the ends of the teeth on the terminal 11.However, the cutter element can also be installed in a position 180 ofrotation from the illustrated position. In such an event, the cutteredge is located adjacent to the ends of the teeth 17 on the terminalelement 12. With this structure, it is merely necessary to place. thecutter element in the body in either of the two positions, and it is notnecessary to insure that it is oriented in one position or the other.This simplifies assembly and reduces cost.

Preferably, a transparent insulating disc 33 formed of high temperatureplastic such as a high temperature Mylar or the like is inserted andretained in the body above the center portion of the terminals. Sincesuch disc is transparent, the condition of the device can be viewed eventhough the disc provides some protection for the operation portion ofthe device. In instances where is is necessary to break two lines of acircuit, a similar device is provided with two separate pairs ofterminals and a separate connector strip to connect each pair. In suchdevices, a single disc operates a single cutter having a cutting edgesufficiently long to engage and rupture both strips.

Because the snap disc provides very rapid operation and greatreliability, the device can safely be used to protect circuits in whichover-temperature can create a hazardous or damaging condition. Since thenumber of elements required to manufacture the device is small, and theelements are capable of being manufactured by high production, low costmanufacturing procedures and since the assembly of the device iseconomical, a low cost, reliable thermal-sensitive fuse device isprovided.

Although a preferred embodiment of this invention is illustrated, it isto be understood that various modifications and rearrangement of partsmay be resorted to without departing from the scope of the inventiondisclosed and claimed herein.

What is claimed is:

l. A thermostat fuse comprising a body, a pair of terminals on saidbody, a frangible conductor electrically connecting said terminals, abimetallic snap disc mounted at its periphery in said body and providinga central portion movable with snap action from a first position to asecond position when the environmental temperature reaches apredetermined temperature, and an operator member supported in said bodymovable by said central portion to rupture said conductor and break theconnection between said terminals when said central portion moves fromsaid first position to said second position.

2. A fuse as set forth in claim 1 wherein said operator member formedwith an end shaped to facilitate the rupture of said conductor, saidoperator member being operatively connected for movement by said snapdisc.

3. A fuse as set forth in claim 2 wherein said operator member is anelongated member formed of a non-conductive material with said one endformed with a relatively sharp cutter and the other end engagable withthe center of said disc.

4. A fuse as set forth in claim 3 wherein said conductor is a thin sheetof metal and at least one terminal includes sharp projections engagablewith said metal on the side opposite thereof from said operator member.

5. A fuse as set forth in claim 4 wherein said operator member and bodyare provided with interengaging guide surfaces operable to maintain apredetermined orientation of said member.

6. A fuse as set forth in claim 1 wherein said snap disc remains in saidsecond position when its temperature returns to normal environmentaltemperatures.

7. A fuse as set in claim 6 wherein said conductor is formed ofa ductilematerial, and said operator member non-resiliently bends the rupturedends of said conductor apart when they move to said operated position.

8. A fuse as set forth in claim 1 wherein said conductor is formed of aductile material, and said operator member non-resiliently bends theruptured ends of said conductor apart when they move to said operatedpositron.

9. A fuse as set forth in claim 1 wherein said conductor is a thin stripof metal, and support means are provided on the side of said conductoropposite said operator member, said support means providing localizedhigh stress in said conductor when said operator member moves to saidoperated position.

10. A fuse as set forth in claim 9 wherein said support means includes aplurality of sawtoothed shaped teeth providing points which producelocalized high stress when said operator member operates.

tially adjacent to one support means and spaced. fromthe other supportmeans.

13. A fuse as set forth in claim 12 wherein said operator member is anelongated cutter engagable with said conductor along said narrow line,said cutter and body being shaped so that said cutter is insertable in Isaid body in either of two positions.

1. A thermostat fuse comprising a body, a pair of terminals on saidbody, a frangible conductor electrically connecting said terminals, abimetallic snap disc mounted at its periphery in said body and providinga central portion movable with snap action from a first position to asecond position when the environmental temperature reaches apredetermined temperature, and an operator member supported in said bodymovable by said central portion to rupture said conductor and break theconnection between said terminals when said central portion moves fromsaid first position to said second position.
 1. A thermostat fusecomprising a body, a pair of terminals on said body, a frangibleconductor electrically connecting said terminals, a bimetallic snap discmounted at its periphery in said body and providing a central portionmovable with snap action from a first position to a second position whenthe environmental temperature reaches a predetermined temperature, andan operator member supported in said body movable by said centralportion to rupture said conductor and break the connection between saidterminals when said central portion moves from said first position tosaid second position.
 2. A fuse as set forth in claim 1 wherein saidoperator member formed with an end shaped to facilitate the rupture ofsaid conductor, said operator member being opEratively connected formovement by said snap disc.
 3. A fuse as set forth in claim 2 whereinsaid operator member is an elongated member formed of a non-conductivematerial with said one end formed with a relatively sharp cutter and theother end engagable with the center of said disc.
 4. A fuse as set forthin claim 3 wherein said conductor is a thin sheet of metal and at leastone terminal includes sharp projections engagable with said metal on theside opposite thereof from said operator member.
 5. A fuse as set forthin claim 4 wherein said operator member and body are provided withinterengaging guide surfaces operable to maintain a predeterminedorientation of said member.
 6. A fuse as set forth in claim 1 whereinsaid snap disc remains in said second position when its temperaturereturns to normal environmental temperatures.
 7. A fuse as set in claim6 wherein said conductor is formed of a ductile material, and saidoperator member non-resiliently bends the ruptured ends of saidconductor apart when they move to said operated position.
 8. A fuse asset forth in claim 1 wherein said conductor is formed of a ductilematerial, and said operator member non-resiliently bends the rupturedends of said conductor apart when they move to said operated position.9. A fuse as set forth in claim 1 wherein said conductor is a thin stripof metal, and support means are provided on the side of said conductoropposite said operator member, said support means providing localizedhigh stress in said conductor when said operator member moves to saidoperated position.
 10. A fuse as set forth in claim 9 wherein saidsupport means includes a plurality of sawtoothed shaped teeth providingpoints which produce localized high stress when said operator memberoperates.
 11. A fuse as set forth in claim 10 wherein said operatormember engages the side of said conductor opposite said support meansalong a narrow line substantially adjacent to said points.
 12. A fuse asset forth in claim 11 wherein similar support means are provided at twolocations spaced along said conductor, and said narrow line issubstantially adjacent to one support means and spaced from the othersupport means.